U.S. Pat. No. 4,507,605, issued Mar. 26, 1985 for a "Method and Apparatus for Electrical and Optical Inspection and Testing of Unpopulated Printed Circuit Boards and Other Like Items"--assigned to Testamatic Corporation of Lathem, N.Y. describes a novel method and apparatus for the electrical and mechanical inspection and testing of unpopulated electronic printed circuit boards, ceramic substrates and other like items which have conductive paths formed thereon together with interconnecting pathways and other conducting surfaces. This method and apparatus employs a low presure gaseous atmosphere contained within a sealed chamber for producing electro-luminescence around the conductive pads, pathways, and other conductive surfaces in the manner described briefly above, and employs a transparent grid of fine conductor wires used in conjunction with a movable probe to produce an electric field across the gaseous atmosphere within the test chamber.
An important feature of this novel method and apparatus for testing unpopulated circuit boards, etc. in the above briefly-discussed manner, is the provision of a high speed, low mass movable probe suitable for use in the sealable test chamber and which is described in U.S. Pat. No. 4,527,119, issued July 2, 1985 for a "High Speed, Low Mass, Movable Probe and/or Instrument Positioner, Tool and Like Items Suitable for Use in a Controlled Environment Chamber", assigned to Testamatic Corporation of Latham, N.Y.
FIG. 1 is a partial plan view of what an observer would see while looking through the transparent partition of the testing equipment described in U.S. Pat. No. 4,507,605 under conditions where a sample test printed circuit board was being tested with the chamber filled with a gaseous vapor and suitable electrical excitation potential applied between the transparent fine wire grid and certain selected conductive pads upon the exterior surface of the test circuit board which are exposed to the low pressure gaseous vapor. As described briefly above and in more detail in U.S. Pat. No. 4,507,605, electro-luminescence will occur within the chamber over those conductive pads, pathways, and other conductive surfaces which are being contacted by the movable probe and all such electrically conductive surfaces having electrical continuity with the pad being contacted by the movable probe and which also have conductive surfaces exposed to the gaseous vapor. A number of such conductive pads which produce this electro-luminescent effect are illustrated at 11 in FIG. 1 by the use of little spurs or short lines projecting outwardly from the surface of such pad to depict light emissions. During the testing of any particular printed circuit board, or the like, it will be known in advance which of the particular pads, such as those illustrated at 11, should be lighted up when the movable probe contacts a selected pad as a result of the particular design of the board under test. By optically scanning the entire surface of the board for each particular setting of the movable probe, using television vidicon, image orthocon, or other TV camera tube or scanning photometer, and comparing the observed pattern to programmed information stored in a control computer, it can be determined whether or not the proper pads are being illuminated for any particular setting of the movable probe. In the event that all of the pads which should but are not electro-luminescing (indicative of a crack or open circuit) or undesired additional pads are electro-luminescing (indicative of the existence of a short), it readily can be determined automatically whether or not faults exist in the particular board being tested. For a more detailed description of the phenomenon whereby automatic testing in this manner employing electro-luminescence is achieved, reference is made to the above-noted U.S. Pat. Nos. 4,507,605 and 4,527,119, the disclosures of which hereby are incorporated in their entirety into the disclosure of this application.
One of the problems that has been encountered in practice with automatic test equipment employing electro-luminescence in the above-briefly described manner, has been the production of undesired background noise illumination within the test chambers as indicated by the somewhat continuous lines of illuminated fine conductor wires such as shown at 12 and 13 in FIG. 1. These illuminated conductor wires 12 and 13 in fact comprise part of the transparent, conductive grid which is used to impress the electric field across the gaseous vapor within the sealable chamber in order to achieve the electro-luminescent effect. Under test conditions where undesired background noise illumination such as illustrated at 12 and 13 is sufficiently bright, the automatically operating electro-optical scanning equipment for reading out the light patterns, may improperly read the undesired noise background illumination as an excited pad or other conductive surface on the test specimen thereby producing a false read-out and impairing the validity of the test.
FIG. 2 is an exploded, partial perspective view of the interior of the test chamber and illustrates a portion of the transparent, fine wire grid, shown generally at 14, which is disposed within the sealable test chamber under the transparent partition. In the known test apparatus, the grid 14 is comprised by a first set of parallel, fine conductor wires 15 having exposed conductive surface and a second set of parallel, fine conductor wires 16, also having exposed conductive surfaces, with the second set 16 being lying transversely at right angles to and spaced apart from the first set of parallel conductor wires 15. As shown in FIG. 2, the transparent conductive grid 14 is disposed over a test specimen printed circuit board 17 having a conductive pad 18 formed thereon which is contacted by the movable probe 19. Upon electrical excitation of the probe 19 with a negative polarity electric potential and excitation of the fine wire grid 14 with a positive polarity potential, an electric field is produced across gaseous vapor (indicated by the cloud 21) contained within the test chamber so as to induce an electro-luminescent light effect over conductive pad 18 (as well as all other exposed conductive pads, runways, etc., having electrical continuity with the pad 18 and also exposed to the gaseous vapor 21.
During operation in the above briefly-described manner, selected ones of either the parallel grid wire set 15, or selected ones of the parallel grid wire set 16 will be electrically excited (indicated as selected grid wires), and the parallel conductor grid wires of the transversely disposed set 15 are not energized (unselected). Because of the above-briefly described arrangement of selected (electrically excited) and unselected (neutral or ground potential) grid wires, an electric potential difference will exist between the selected grid wires and the transversely arrayed unselected grid wires which pass under them as indicated by the electric field lines 22 in FIG. 2. Those transversely arrayed ground potential (unselected) grid wires 15 which pass under the excited (selected) grid wires can and do become electrically charged through the gaseous media, and if the phenomenon is sufficient to induce a significant charge on the thus coupled unselected grid wires, then the effect illustrated at 12 and 13 in FIG. 1 and described briefly in the preceeding paragraphs, occurs. The occurrance of this undesired noise background illumination thus obscures and adversely affects the results of the automatic test being conducted. In order to reduce and obviate the effects of such undesired background noise illumination in an electro-luminescent type of automatic testing equipment, the present invention was devised.